The mutation in genes happens only in one in 10,000 people and a search showed no other case of it being documented in only one identical twin in the world, according to experts from the Center for DNA Typing of the Fourth Military Medical University.

Local police looking for a missing girl in Qishan County in June discovered she had been raped and murdered.

Police located a suspect in the girl's neighborhood. His DNA matched genetic material at the crime scene, and the suspect confessed, police said.

However, the suspect had an identical twin brother who also lived in the neighborhood.

Prosecutors said police needed to rule out involvement of the brother, who is younger by a few minutes.

"For judicial fairness, we must confirm which of the twins conducted the crime, or if both did, and find other evidence," said Li Hongxu, vice director of Qishan County Police Bureau. "If the technical methods don't work, we must have other evidence such as witnesses."

"It is the first time that we receive such a case," said Wu Yuanming, the DNA center director. "It is a difficult task to identify differences in identical twins anywhere."

Scientists had a surprising finding: The younger brother was found to have the rare mutation of a gene, which didn't happen to the elder brother.

Samples from the scene confirmed the elder brother was the suspect, police said.